Add Variable Length Quantity

config.json

@@ -1059,6 +1059,14 @@
         "prerequisites": [],
         "difficulty": 4
       },
+      {
+        "slug": "variable-length-quantity",
+        "name": "Variable Length Quantity",
+        "uuid": "d0ef5012-0034-4a71-a8a8-e2ed99f93d70",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
+      },
       {
         "slug": "rectangles",
         "name": "Rectangles",

exercises/practice/variable-length-quantity/.docs/instructions.md

@@ -0,0 +1,34 @@
+# Instructions
+
+Implement variable length quantity encoding and decoding.
+
+The goal of this exercise is to implement [VLQ][vlq] encoding/decoding.
+
+In short, the goal of this encoding is to encode integer values in a way that would save bytes.
+Only the first 7 bits of each byte are significant (right-justified; sort of like an ASCII byte).
+So, if you have a 32-bit value, you have to unpack it into a series of 7-bit bytes.
+Of course, you will have a variable number of bytes depending upon your integer.
+To indicate which is the last byte of the series, you leave bit #7 clear.
+In all of the preceding bytes, you set bit #7.
+
+So, if an integer is between `0-127`, it can be represented as one byte.
+Although VLQ can deal with numbers of arbitrary sizes, for this exercise we will restrict ourselves to only numbers that fit in a 32-bit unsigned integer.
+Here are examples of integers as 32-bit values, and the variable length quantities that they translate to:
+
+```text
+ NUMBER        VARIABLE QUANTITY
+00000000              00
+00000040              40
+0000007F              7F
+00000080             81 00
+00002000             C0 00
+00003FFF             FF 7F
+00004000           81 80 00
+00100000           C0 80 00
+001FFFFF           FF FF 7F
+00200000          81 80 80 00
+08000000          C0 80 80 00
+0FFFFFFF          FF FF FF 7F
+```
+
+[vlq]: https://en.wikipedia.org/wiki/Variable-length_quantity

exercises/practice/variable-length-quantity/.meta/config.json

@@ -0,0 +1,17 @@
+{
+  "authors": ["ryanplusplus"],
+  "files": {
+    "solution": [
+      "src/variable_length_quantity.cr"
+    ],
+    "test": [
+      "spec/variable_length_quantity_spec.cr"
+    ],
+    "example": [
+      ".meta/src/example.cr"
+    ]
+  },
+  "blurb": "Implement variable length quantity encoding and decoding.",
+  "source": "A poor Splice developer having to implement MIDI encoding/decoding.",
+  "source_url": "https://splice.com"
+}

exercises/practice/variable-length-quantity/.meta/src/example.cr

@@ -0,0 +1,30 @@
+module VariableLengthQuantity
+  def self.decode(bytes : Array(Number))
+    values = [] of UInt32
+    n : UInt32 = 0
+    bytes.each_with_index do |byte, i|
+      n = n << 7
+      n = n + (byte & 0x7f)
+      if byte & 0x80 == 0
+        values << n
+        n = 0
+      end
+      raise ArgumentError.new unless (i != bytes.size - 1) || (byte & 0x80 == 0)
+    end
+    values
+  end
+
+  def self.encode(values : Array(Number))
+    values.flat_map { |value| encode(value) }
+  end
+
+  private def self.encode(n : Number)
+    bytes = [n & 0x7f]
+    n = n >> 7
+    while n > 0
+      bytes << (n & 0x7f | 0x80)
+      n = n >> 7
+    end
+    bytes.reverse
+  end
+end

exercises/practice/variable-length-quantity/.meta/test_template.ecr

@@ -0,0 +1,22 @@
+require "spec"
+require "../src/*"
+
+describe <%= to_capitalized(@json["exercise"].to_s).inspect %> do
+    <% @json["cases"].as_a.each do |cases| %>
+        describe <%= cases["description"].inspect %> do
+            <%- cases["cases"].as_a.each do |cases| %>
+                <%= status()%> "<%-= cases["description"] %>" do
+                    <% if cases["expected"].as_h? %>
+                        expect_raises(ArgumentError) do
+                            <%= to_capitalized(@json["exercise"].to_s) %>.<%= cases["property"] %>(<%= cases["input"]["integers"].as_a.inspect %>)
+                        end
+                    <% else %>
+                        input = <%= cases["input"]["integers"].as_a.inspect %>
+                        expected = <%= cases["expected"].as_a.inspect %>
+                        <%= to_capitalized(@json["exercise"].to_s) %>.<%= cases["property"] %>(input).should eq(expected)
+                    <% end %>
+                end
+            <% end %>
+        end
+    <% end %>
+end

exercises/practice/variable-length-quantity/.meta/tests.toml

@@ -0,0 +1,88 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[35c9db2e-f781-4c52-b73b-8e76427defd0]
+description = "Encode a series of integers, producing a series of bytes. -> zero"
+
+[be44d299-a151-4604-a10e-d4b867f41540]
+description = "Encode a series of integers, producing a series of bytes. -> arbitrary single byte"
+
+[ea399615-d274-4af6-bbef-a1c23c9e1346]
+description = "Encode a series of integers, producing a series of bytes. -> largest single byte"
+
+[77b07086-bd3f-4882-8476-8dcafee79b1c]
+description = "Encode a series of integers, producing a series of bytes. -> smallest double byte"
+
+[63955a49-2690-4e22-a556-0040648d6b2d]
+description = "Encode a series of integers, producing a series of bytes. -> arbitrary double byte"
+
+[29da7031-0067-43d3-83a7-4f14b29ed97a]
+description = "Encode a series of integers, producing a series of bytes. -> largest double byte"
+
+[3345d2e3-79a9-4999-869e-d4856e3a8e01]
+description = "Encode a series of integers, producing a series of bytes. -> smallest triple byte"
+
+[5df0bc2d-2a57-4300-a653-a75ee4bd0bee]
+description = "Encode a series of integers, producing a series of bytes. -> arbitrary triple byte"
+
+[f51d8539-312d-4db1-945c-250222c6aa22]
+description = "Encode a series of integers, producing a series of bytes. -> largest triple byte"
+
+[da78228b-544f-47b7-8bfe-d16b35bbe570]
+description = "Encode a series of integers, producing a series of bytes. -> smallest quadruple byte"
+
+[11ed3469-a933-46f1-996f-2231e05d7bb6]
+description = "Encode a series of integers, producing a series of bytes. -> arbitrary quadruple byte"
+
+[d5f3f3c3-e0f1-4e7f-aad0-18a44f223d1c]
+description = "Encode a series of integers, producing a series of bytes. -> largest quadruple byte"
+
+[91a18b33-24e7-4bfb-bbca-eca78ff4fc47]
+description = "Encode a series of integers, producing a series of bytes. -> smallest quintuple byte"
+
+[5f34ff12-2952-4669-95fe-2d11b693d331]
+description = "Encode a series of integers, producing a series of bytes. -> arbitrary quintuple byte"
+
+[7489694b-88c3-4078-9864-6fe802411009]
+description = "Encode a series of integers, producing a series of bytes. -> maximum 32-bit integer input"
+
+[f9b91821-cada-4a73-9421-3c81d6ff3661]
+description = "Encode a series of integers, producing a series of bytes. -> two single-byte values"
+
+[68694449-25d2-4974-ba75-fa7bb36db212]
+description = "Encode a series of integers, producing a series of bytes. -> two multi-byte values"
+
+[51a06b5c-de1b-4487-9a50-9db1b8930d85]
+description = "Encode a series of integers, producing a series of bytes. -> many multi-byte values"
+
+[baa73993-4514-4915-bac0-f7f585e0e59a]
+description = "Decode a series of bytes, producing a series of integers. -> one byte"
+
+[72e94369-29f9-46f2-8c95-6c5b7a595aee]
+description = "Decode a series of bytes, producing a series of integers. -> two bytes"
+
+[df5a44c4-56f7-464e-a997-1db5f63ce691]
+description = "Decode a series of bytes, producing a series of integers. -> three bytes"
+
+[1bb58684-f2dc-450a-8406-1f3452aa1947]
+description = "Decode a series of bytes, producing a series of integers. -> four bytes"
+
+[cecd5233-49f1-4dd1-a41a-9840a40f09cd]
+description = "Decode a series of bytes, producing a series of integers. -> maximum 32-bit integer"
+
+[e7d74ba3-8b8e-4bcb-858d-d08302e15695]
+description = "Decode a series of bytes, producing a series of integers. -> incomplete sequence causes error"
+
+[aa378291-9043-4724-bc53-aca1b4a3fcb6]
+description = "Decode a series of bytes, producing a series of integers. -> incomplete sequence causes error, even if value is zero"
+
+[a91e6f5a-c64a-48e3-8a75-ce1a81e0ebee]
+description = "Decode a series of bytes, producing a series of integers. -> multiple values"

exercises/practice/variable-length-quantity/spec/variable_length_quantity_spec.cr

@@ -0,0 +1,164 @@
+require "spec"
+require "../src/*"
+
+describe "VariableLengthQuantity" do
+  describe "Encode a series of integers, producing a series of bytes." do
+    it "zero" do
+      input = [0]
+      expected = [0]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "arbitrary single byte" do
+      input = [64]
+      expected = [64]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "largest single byte" do
+      input = [127]
+      expected = [127]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "smallest double byte" do
+      input = [128]
+      expected = [129, 0]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "arbitrary double byte" do
+      input = [8192]
+      expected = [192, 0]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "largest double byte" do
+      input = [16383]
+      expected = [255, 127]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "smallest triple byte" do
+      input = [16384]
+      expected = [129, 128, 0]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "arbitrary triple byte" do
+      input = [1048576]
+      expected = [192, 128, 0]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "largest triple byte" do
+      input = [2097151]
+      expected = [255, 255, 127]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "smallest quadruple byte" do
+      input = [2097152]
+      expected = [129, 128, 128, 0]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "arbitrary quadruple byte" do
+      input = [134217728]
+      expected = [192, 128, 128, 0]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "largest quadruple byte" do
+      input = [268435455]
+      expected = [255, 255, 255, 127]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "smallest quintuple byte" do
+      input = [268435456]
+      expected = [129, 128, 128, 128, 0]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "arbitrary quintuple byte" do
+      input = [4278190080]
+      expected = [143, 248, 128, 128, 0]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "maximum 32-bit integer input" do
+      input = [4294967295]
+      expected = [143, 255, 255, 255, 127]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "two single-byte values" do
+      input = [64, 127]
+      expected = [64, 127]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "two multi-byte values" do
+      input = [16384, 1193046]
+      expected = [129, 128, 0, 200, 232, 86]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+
+    pending "many multi-byte values" do
+      input = [8192, 1193046, 268435455, 0, 16383, 16384]
+      expected = [192, 0, 200, 232, 86, 255, 255, 255, 127, 0, 255, 127, 129, 128, 0]
+      VariableLengthQuantity.encode(input).should eq(expected)
+    end
+  end
+
+  describe "Decode a series of bytes, producing a series of integers." do
+    pending "one byte" do
+      input = [127]
+      expected = [127]
+      VariableLengthQuantity.decode(input).should eq(expected)
+    end
+
+    pending "two bytes" do
+      input = [192, 0]
+      expected = [8192]
+      VariableLengthQuantity.decode(input).should eq(expected)
+    end
+
+    pending "three bytes" do
+      input = [255, 255, 127]
+      expected = [2097151]
+      VariableLengthQuantity.decode(input).should eq(expected)
+    end
+
+    pending "four bytes" do
+      input = [129, 128, 128, 0]
+      expected = [2097152]
+      VariableLengthQuantity.decode(input).should eq(expected)
+    end
+
+    pending "maximum 32-bit integer" do
+      input = [143, 255, 255, 255, 127]
+      expected = [4294967295]
+      VariableLengthQuantity.decode(input).should eq(expected)
+    end
+
+    pending "incomplete sequence causes error" do
+      expect_raises(ArgumentError) do
+        VariableLengthQuantity.decode([255])
+      end
+    end
+
+    pending "incomplete sequence causes error, even if value is zero" do
+      expect_raises(ArgumentError) do
+        VariableLengthQuantity.decode([128])
+      end
+    end
+
+    pending "multiple values" do
+      input = [192, 0, 200, 232, 86, 255, 255, 255, 127, 0, 255, 127, 129, 128, 0]
+      expected = [8192, 1193046, 268435455, 0, 16383, 16384]
+      VariableLengthQuantity.decode(input).should eq(expected)
+    end
+  end
+end

exercises/practice/variable-length-quantity/src/variable_length_quantity.cr

@@ -0,0 +1,3 @@
+module VariableLengthQuantity
+  # Write your code for the 'Variable Length Quantity' exercise in this file.
+end